When we call for a cookie sheet in our recipes, we mean the kind of baking sheet that doesn’t have a rim all the way around it, shown at far right in the photo. A cookie sheet has at least one and as many as three rimless sides, which aid hot-air flow to the cookies, as well as make it easier to remove the cookies from the sheet once they’re baked.
In many instances, using a rimmed baking sheet (also known as a half sheet pan or a jellyroll pan, shown at near right) instead of a cookie sheet won’t make any difference, but sometimes it does. Sugar cookies, for instance, don’t brown as evenly on a rimmed sheet. So for the very best results from our baking recipes, use a cookie sheet when one is called for.